Dust in the wind helped bring down CO2 in the past

Atmospheric simulation of aerosol transport in December, 2006. Dust is shown in red.

William M. Putman and Arlindo M. da Silva, NASA/GISS

Over the past three million years, the Earth has experienced rhythmic swings in and out of glacial conditions paced by a drummer named Milankovitch. The Milankovitch cycles in Earth’s orbit around the sun change the way the sun’s light strikes the surface of our planet. But these climatic swings would have been much smaller if not for the amplifying effect of changing greenhouse gas concentrations. (Returning to a band metaphor, a rock concert would be pretty unremarkable if you disconnected the amps from the guitars.)

Climate records like ice cores very neatly show us how those concentrations changed over time, since they hold bubbles of ancient air trapped in the ice. But it’s up to us to figure out why greenhouse gasses moved in to and out of the atmosphere. For example, where did all the atmospheric CO2 go when the warm interglacials descended back into glacial conditions?

Hiding carbon in the ocean

The Southern Ocean has been the prime suspect. There, CO2-rich deep ocean water rises to the surface and exchanges gas with the atmosphere. If that ventilation were too slow, atmospheric CO2 would fall. Reduced upwelling caused by a lid of lower-density water near the Antarctic coast, for example, could explain up to 40 parts per million of the approximately 100 parts per million decrease in CO2 over the recent glaciations.

That still leaves a lot to be accounted for, though. In the late 1980s, oceanographers solved a nagging puzzle. They had discovered regions of the ocean where plenty of critical nutrients—nitrogen and phosphorus—were present, but photosynthetic productivity was low. What was holding phytoplankton back? A limited supply of iron.

Iron in airborne dust can be carried long distances from arid regions; when it lands in the ocean, it turns out to fuel the growth of marine phytoplankton.

John H. Martin and colleagues proposed that this could help explain where some of the CO2 went during the glacial periods. If more dust, and thus more iron, got into the oceans, then enhanced biological activity could draw carbon into the deep ocean.

Antarctic ice cores contain larger amounts of airborne dust during the glacial periods, much of which is thought to have come from the Patagonian region of South America. The great plain of sediment washing out from the melting end of a glacier is an ideal source of dust for the wind to whisk away. This is especially true for Patagonia, where the winds are strong, and the rain shadow would have been even more pronounced during glacial periods. Grow bigger glaciers there, and you get more airborne dust blowing across the Southern Ocean.

The growth of phytoplankton “fertilized” by all that iron would transport carbon from the atmosphere to the deep ocean. Phytoplankton, of course, take in CO2 as a part of photosynthesis, gaining energy and the material to build cells. When they die and sink to the bottom (either solo or via the express fecal route courtesy of larger creatures that made a meal of them), they take that carbon with them.

Great idea, how do you test it?

Here’s where the rubber meets the road: how do you test whether that actually happened? There have been attempts over the years, but the results have been muddy. They’ve mostly relied on the fact that phytoplankton are a little more likely to use nitrate molecules containing nitrogen-14 atoms (the most common isotope) than nitrogen-15. The precise ratio of nitrogen-15 to nitrogen-14 in phytoplankton depends on how much nitrate is available—if there's a shortage, then any isotope will do. If an iron-limited region of the ocean is fertilized with airborne dust, more of the nitrate will be used up, and the concentration will decrease. So the ratio of nitrogen isotopes (which can be recorded in sediment cores) tells us about how much nitrate was being used.

A new study led by Alfredo Martínez-García at ETH Zürich provides the best test of the iron fertilization hypothesis yet. Technical advances allowed the researchers to measure nitrogen isotopes in the calcium carbonate shells of plankton called foraminifera in a core of sediment from the seafloor. Older studies analyzed diatoms or the sediment itself, both of which were fraught with complicating factors that made the results difficult to interpret. The researchers also extracted records of photosynthetic productivity and iron from wind-blown dust from the core, which spanned the last 160,000 years.

The correlation between the nitrogen isotopes and iron was quite strong. Iron increased as the climate cooled into the last glaciation, consistent with being downwind of the Patagonian dust source, and the concentration of nitrate in the surface ocean appears to have decreased in concert. The productivity record, too, showed higher levels of photosynthesis during those periods.

It’s possible that some of this change in nitrate was due to shifts in surface water movement or the amount of nitrate carried into the region, but the researchers believe those effects to be minor. Overall, the evidence points to iron fertilization having a clear impact, which would have resulted in more carbon from the atmosphere being transferred to the deep ocean. That could have accounted for another 40 parts per million of the drawdown in CO2 during the glaciation.

The same process also appears to have been at work on shorter timescales, contributing to changes in CO2 over smaller climate fluctuations that only lasted a few thousand years.

Records like this help clarify the Southern Ocean’s role among the various parts of the climate system that translated the rhythm of the Milankovitch orbital cycles into significant climate changes.